1. Field of the Invention
The present invention relates to an imaging device capable of imaging objects in the omniazimuthal angle of 360 degrees and used in the fields of visual systems such as, for example, surveillance cameras; and a method for producing the same.
2. Description of the Related Art
Recently in the fields of visual systems, such as, for example, surveillance cameras, various attempts have been made to allow a camera to perform monitoring operations conventionally conducted by the human eye, by combining the camera with a computer.
A generally used camera, which has a limited viewing angle, is not suitable for such applications. Therefore, cameras using fish-eye lenses or other wide-angle lenses have been developed. For example, in the field of movable robots, the use of convex mirrors having a shape of solid of revolution (such as conical mirrors), spherical mirrors or the like, have been actively studied. (Hereinafter, such convex mirrors will be referred to as “convex rotatable mirrors”.) According to systems studied in this field, an optical image of a viewing angle of 360 degrees is taken by a convex rotatable mirror, then the optical image is converted into a video image, and the video image is further converted into a desirable image by a computer.
FIG. 8A shows a schematic structure of an imaging device 80 using a conventional convex rotational mirror, and FIG. 8B is a cross-sectional view thereof taken along a plane including the rotation axis of a convex rotational mirror 93 described below.
The imaging device 80 includes a convex rotational mirror unit 91. The convex rotational mirror unit 91 includes a generally disc-shaped base 92 and the generally conical convex rotational mirror 93. The imaging device 80 further includes a generally cup-shaped, spherical optical member 94. The optical member 94 is open toward the convex rotational mirror unit 91 and covers a surface of the base 92 and the convex rotational mirror 93. The optical member 94 holds the convex rotational mirror unit 91 and is formed of a light-transmissive material. An inner circumferential surface of the optical member 94 and the convex rotational mirror 93 interpose a hollow space therebetween. The optical member 94 has a thickness which is sufficiently thin to allow light which is incident on an outer circumferential surface of the optical member 94 to be transmitted through the optical member 94, so that it is proximately parallel to light which is directed toward the convex rotational mirror 93 from the inner circumferential surface of the optical member 94.
A generally cylindrical imaging mechanism 98 is provided on the opposite side to the convex rotational mirror unit 91, with the optical member 94 interposed therebetween. The imaging mechanism 98 is able to be engaged with the optical member 94. The imaging mechanism 98 includes a lens 99 facing an opening of the optical member 94, which is formed on the opposite side to the convex rotational mirror unit 91, and an imaging section 90 provided on the opposite side to the optical member 94, with the lens 99 interposed therebetween. The imaging section 90 is connected to a signal processing section 88 provided for adjusting the distortion of an image taken by the imaging mechanism 98.
As described above, the light-transmissive optical member 94 is used for holding the convex rotational mirror unit 91, and thus a separate holding member is not provided. The reason is that if a separate holding member is provided for holding the convex rotational mirror unit 91, an image of the holding member itself would be taken and so would be a part of an image taken by the imaging mechanism 98.
As the convex rotational mirror 93, a mirror which reflects incident light at a surface thereof is used so that an image formed by the reflection does not overlap the image that is intended to be taken. The convex rotational mirror 93 is formed of a metal material. Alternatively, the convex rotational mirror 93 may have a metal material, for example, vapor-deposited or plated onto an outer circumferential surface thereof.
The imaging device 80 having the above-described structure operates as follows.
Light 71 is incident on the outer circumferential surface of the light-transmissive optical member 94 and is transmitted through the optical member 94. While being transmitted through the optical member 94, the incident light 71 is refracted twice (not shown) so as to become light 72. The light 72 is directed from the inner circumferential surface of the optical member 94 toward the convex rotational mirror 93 through the hollow space between the optical member 94 and the convex rotational mirror 93. Then, the light 72 is reflected by the convex rotational mirror 93 and is directed toward the imaging mechanism 98 as reflected light 81. The reflected light 81 is transmitted through the lens 99 of the imaging mechanism 98 and incident on the imaging section 90. The imaging section 90 transforms the reflected light 81 into an image signal representing an image and outputs the image signal to the signal processing section 88. The signal processing section 88 processes the received image signal so as to adjust the distortion of the image.
The conventional imaging device 80 has the following problems.
(1) An outer surface of the convex rotational mirror 93 is exposed to the air filling the hollow space between the optical member 94 and the convex rotational mirror 93. Therefore, the metal on the surface of the convex rotational mirror 93 is oxidized, or the metal material vapor-deposited or plated onto the surface of the convex rotational mirror 93 is exfoliated. As a result, the reflectance of the light incident on the convex rotational mirror 93 is lowered.
(2) Dust, moisture or the like which invades into the hollow space between the optical member 94 and the convex rotational mirror 93 through the gap between the base 92 and the optical member 94, causes noise to the light 72 and the reflected light 81. Therefore, the quality of the image taken by the imaging mechanism 98 is degraded.
When the imaging device 80 having the above-described problem is used outdoors or the like for an extended period of time, external factors such as temperature, moisture, ultraviolet rays or the like cause the following problems. The metal material of the convex rotational mirror 93 is oxidized and thus corroded. When the metal material is vapor-deposited or plated onto the surface of the convex rotational mirror 93, such metal material is exfoliated or corroded. It is necessary to prevent exfoliation and corrosion of the metal material and also to prevent dust or moisture from entering the connection part of the optical member 94 and the convex rotational mirror 93.
(3) The optical member 94 is hollow and therefore is mechanically weak and easy to break. In actual use, specific care needs to be taken to avoid breakage.